1. Field of the Invention
The present invention relates to data transfer through a communication system channel, and, more particularly, to signal adjustment for detection of data information from a recording medium.
2. Description of the Related Art
A read channel component is an integrated circuit (IC) of a computer hard disk (HD) drive that encodes, detects, and decodes data, enabling a read/write head to correctly i) write data to the disk drive and ii) read back the data. The disks in an HD drive have a number of tracks, each track consisting of i) user (or “read”) data sectors and ii) control (or “servo”) data sectors embedded between the read sectors. Information stored in the servo sectors is employed to position the head (e.g., a magnetic recording/playback head) over a track so that the information stored in the read sector can be retrieved properly.
A servo sector typically comprises a servo preamble, an encoded servo address mark (SAM), encoded Gray data, a burst demodulation (demod) field, and a repeatable run-out (RRO) field. The servo preamble allows for timing recovery and gain adjustment of the written servo data. The SAM is an identifier of fixed bit-length that identifies the data as servo data, with the value for this identifier the same for all servo sectors. Gray data represents the track number/cylinder information and provides coarse positioning information for the head. The burst demod field provides fine positioning information for the head. RRO field data provides head positioning information that is i) finer than that provided by Gray data and ii) coarser than that provided by the burst demodulation fields. Specifically, RRO field data is typically employed to compensate for when the head does not follow a circular track around the disk. The read sector comprises a read preamble, a read address mark (RAM), and encoded user data. The read preamble also provides for timing recovery and gain adjustment, and the RAM identifies the read sector user data.
When the head of a recording system reads data from a sector of an HD, the data is provided as an analog signal (readback signal) that is subsequently level-adjusted, equalized, and sampled for further digital signal processing to detect and decode the sector information. Some older prior-art systems may perform analog rather than digital signal processing of the read signal. For level-adjustment, prior-art read channel components typically employ one or more attenuator stages and a variable gain amplifier (VGA) to adjust the readback signal provided from the head to a desired level before further processing.
The one or more attenuators provide coarse level-adjustment of the readback signal by attenuation, while the VGA provides fine level-adjustment through variable signal gain. The VGA has a finite range of operation. If the attenuator settings are too high or too low, the VGA may not be able to compensate for the gain error introduced by incorrect attenuator settings and may saturate at the VGA lower or upper gain (or “rail”).
At the beginning of a read or servo event (i.e., when the read or servo sector is being read), the prior-art read channel component performs a zero gain start (ZGS) to quickly predict the gain error in the readback signal prior to acquisition of the readback signal's gain and timing. Signal timing and gain acquisition over the preamble field is referred to as the “acquire mode” (ACQ mode). Since the preamble pattern (e.g., 2T pattern of 11001100. . . ) is known over a preamble field, prior-art systems employ efficient decision-directed (DD) algorithms for acquisition of gain and timing over this field, and using a substantial number of the preamble bits for this acquisition. The gain adjustment is estimated by a ZGS circuit that is provided entirely to a VGA register (which sets the VGA gain) so that adaptive gain adjustment of the DD algorithm starts from a relatively close starting point (i.e., the DD algorithm starts with an almost correct VGA gain as predicted by the ZGS circuitry).
After the ZGS, gain of the VGA is determined automatically (within its finite range of operation) by the read channel component using an adaptive signal-processing algorithm. However, prior-art setting of the attenuation is not an adaptive process. HD drive manufacturers determine off-line an appropriate level of attenuation for different sectors of an HD (a time-consuming and cumbersome process), and then program the read channel's attenuation setting for different sectors in firmware. Pre-programming attenuation levels provides for several problems in prior-art systems.
For example, when a “head switch” occurs (e.g., when a different head in a different disk platter is selected for reading/writing the data), large gain error (e.g., +/−12 dB) from the nominal readback signal amplitudes can occur. Also, the VGA operates best within a linear range of operation near the center of its gain range. VGA performance deteriorates due to non-linear effects when the VGA operates near its lower and upper range levels.
In addition, the ZGS adjustment is dependent on the attenuator setting. For example, for a particular attenuator setting of −12 dB, VGA adjustment range of 0 to 24 dB, and VGA gain setting at the range center (12 dB), a ZGS adjustment of −12 dB adds −12 db to the VGA register which puts the VGA register at its lower limit of 0 dB. Consequently, this ZGS adjustment drives the operation of the VGA to the lower rail. This indicates that the selected attenuation is not adequate, and the incoming signal amplitude is very high.